Energy efficient synthesis of boranes

ABSTRACT

The reaction of halo-boron compounds (B—X compounds, compounds having one or more boron-halogen bonds) with silanes provides boranes (B—H compounds, compounds having one or more B—H bonds) and halosilanes. Inorganic hydrides, such as surface-bound silane hydrides (Si—H) react with B—X compounds to form B—H compounds and surface-bound halosilanes. The surface bound halosilanes are converted back to surface-bound silanes electrochemically. Halo-boron compounds react with stannanes (tin compounds having a Sn—H bond) to form boranes and halostannanes (tin compounds having a Sn—X bond). The halostannanes are converted back to stannanes electrochemically or by the thermolysis of Sn-formate compounds. When the halo-boron compound is BCl 3 , the B—H compound is B 2 H 6 , and where the reducing potential is provided electrochemically or by the thermolysis of formate.

RELATED APPLICATIONS

This application claims the benefit of copending U.S. Provisional PatentApplication No. 60/771,739, filed Feb. 8, 2006 and entitled “EnergyEfficient Synthesis of Boranes,” which is incorporated by referenceherein.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.DE-AC51-06NA25396 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to hydrogen storage, and moreparticularly to an energy efficient synthesis of boranes (boroncompounds having at least one B—H bond).

BACKGROUND OF THE INVENTION

Hydrogen (H₂) is currently the leading candidate for a fuel to replacegasoline/diesel fuel in powering the nation's transportation fleet.There are a number of difficulties and technological barriers associatedwith hydrogen that must be solved in order to realize this “hydrogeneconomy”. Inadequate storage systems for on-board transportationhydrogen are recognized as a major technological barrier (see, forexample, “The Hydrogen Economy: Opportunities, Costs, Barriers, and R&DNeeds,” National Academy of Engineering (NAE), Board on Energy andEnvironmental Systems, National Academy Press (2004).

One of the general schemes for storing hydrogen relates to using achemical compound or system that undergoes a chemical reaction to evolvehydrogen as a reaction product. In principle, this chemical storagesystem is attractive, but systems that have been developed to dateinvolve either: (a) hydrolysis of high-energy inorganic compounds wherethe evolution of hydrogen is very exothermic (sodium borohydride/wateras in the Millennium Cell's HYDROGEN ON DEMAND®, and lithium (ormagnesium) hydride as in SAFE HYDROGEN®, for example), thus making thecost of preparing the inorganic compound(s) high and life-cycleefficiency low; or (b) dehydrogenation of inorganic hydride materials(such as Na₃AlH₆/NaAlH₄, for example) that release hydrogen when warmedbut that typically have inadequate mass storage capacity and inadequaterefueling rates.

Inorganic compounds referred to in (a), above, produce hydrogenaccording to the chemical reactionMH_(x)+XH₂O→M(OH)_(x)+XH₂  (1)where MH_(x) is a metal hydride, and M(OH)_(x) is a metal hydroxide.This reaction is irreversible.

Inorganic hydride materials referred to in (b), above, produce hydrogenaccording to the following chemical reaction, which is reversible withH₂ (hydrogen gas):MH_(X)=M+x/2H₂  (2)where MH_(x) is a metal hydride, M is metal and H₂ is hydrogen gas. Bycontrast to the first reaction, which is irreversible with H₂, thesecond reaction is reversible with H₂.

A practical chemical system that evolves hydrogen yet does not sufferthe aforementioned inadequacies would be important to the plannedtransportation sector of the hydrogen economy. This same practicalchemical system would also be extremely valuable for non-transportationH₂ fuel cell systems, such as those employed in laptop computers andother portable electronic devices, and in small mechanical devices suchas lawnmowers where current technology causes significant pollutionconcerns.

Any heat that must be input to evolve the hydrogen represents an energyloss at the point of use, and any heat that is evolved along with thehydrogen represents an energy loss where the chemical storage medium isregenerated. Either way, energy is lost, which diminishes the life-cycleefficiency. For most organic compounds, such as in those shown inequations 3-5 below, hydrogen evolution reactions are very endothermic,and the compounds are incompetent to evolve hydrogen at ambienttemperature (i.e. thermodynamically incapable of evolving H₂ atsignificant pressure at ambient temperature). For temperatures less thanabout 250-400 degrees Celsius, the equilibrium pressure of hydrogen overmost organic compounds is very small. As a consequence, most commonorganic compounds require heating above about 250 degrees Celsius, andthe continual input of high-grade heat to maintain this temperature, inorder to evolve hydrogen at a useful pressure. CH₄ → C + 2H₂ ΔH⁰ = +18kcal/mol (3) ΔG⁰ = +12 kcal/mol 6CH₄ → cyclohexane + 6H₂ ΔH⁰ = +69kcal/mol (4) ΔG⁰ = +78 kcal/mol cyclohexane → benzene + 3H₂ ΔH⁰ = +49kcal/mol (5) ΔG⁰ = +23 kcal/mol

Most organic compounds have hydrogen evolution reactions that areendergonic (i.e. having a net positive standard free energy of reactionchange, i.e. ΔG⁰>0) and their ambient temperature equilibrium hydrogenpressure is very low, practically unobservable. Thus, most organiccompounds are unsuitable for hydrogen storage, based on both life-cycleenergy efficiency and delivery pressure considerations. Decalin, forexample, evolves hydrogen to form naphthalene when heated to about 250degrees Celsius in the presence of a catalyst (see, for example,Hodoshima et al. in “Catalytic Decalin Dehydrogenation/NaphthaleneHydrogenation Pair as a Hydrogen Source for Fuel-Cell Vehicle,” Int. J.Hydrogen Energy (2003) vol. 28, pp. 1255-1262, incorporated by referenceherein). Hodoshima et al. use a superheated “thin film” reactor thatoperates at a temperature of at least 280 degrees Celsius to producehydrogen from decalin at an adequate rate and pressure. Thus, thisendothermic hydrogen evolution reaction requires both a complexapparatus and high-grade heat, which diminishes the life-cycle energyefficiency for hydrogen storage.

Boranes have high hydrogen storage capacities and have attractedinterest for use as hydrogen storage materials for transportation, butthe difficulty of manufacturing borane compounds, and the life-cycleenergy inefficiency of the chemical process presently used for theirmanufacture, has prevented their widespread use.

Owing to its commercial availability, NaBH₄ (sodium borohydride) is astarting material typically used to prepare borane compounds. Diborane(B₂H₆), for example, is prepared by reacting NaBH₄ with BF₃. Borohydridecompounds (i.e. compounds containing the BH₄ anion or other anionic B—Hgroups) are generally prepared by reacting alkoxyborates with activemetal hydrides e.g. NaH or NaAlH₄. Sodium borohydride itself (NaBH₄),for example, is commercially prepared using the known Schlessingerprocess, which involves reacting sodium hydride (NaH) withtrimethoxyboron (B(OCH₃)₃). While convenient to practice on a small orintermediate laboratory or commercial scale, these reactions are notenergy-efficient; the reaction of NaH with B(OCH₃)₃ is exothermic, andNaH is itself formed in the exothermic reaction of Na metal with H₂, sooverall, about 22 kcal of heat are released per B—H bond that is formed.

Other means are known for forming B₂H₆. The best known is the reactionof BCl₃ with H₂ at high temperature to make BHCl₂ and HCl. Significantequilibrium conversion is possible only if the temperature is on theorder of about 600 degrees Celsius or more, and the product mixture mustbe rapidly quenched, typically within a few seconds, to a temperaturebelow about 100 degrees Celsius to allow BHCl₂ to disproportionate toB₂H₆ and BCl₃. The quenched mixture must be separated rapidly before theB₂H₆ back reacts with the HCl coproduct. BCl₃ and HCl are both highlycorrosive. Their corrosive properties in combination with thedifficulties of heat management make this process costly to practice.

Presently, there is no energy efficient means available for preparingboranes.

Methods and systems that employ chemical compounds for storing andevolving hydrogen at ambient temperature with minimal heat input remainhighly desirable.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention includes a methodfor synthesizing a BH₃-containing compound. The method involvessynthesizing at least one halo-boron compound from a boron-containingprecursor; reacting the at least one halo-boron compound with aninorganic hydride material, thereby generating at least one B—Hcompound; and disproportionating the at least one B—H compound to atleast one BH₃-containing compound.

The invention also includes a method for making a metal hydride,comprising thermolyzing a reactive metal formate. The reactive metalhydride may be a compound of the formula(R′)_(n)(R″)_(m)(X)_(3-n-m)Sn(H)wherein R′ is alkyl; wherein R″ is aryl or aryl attached to a polymerbackbone; wherein X is F, Cl, Br, or I; wherein n is 0, 1, 2, or 3;wherein m is 0, 1, 2, or 3; and wherein n+m≦3.

The invention also includes a method of forming BH₃NH₃ and relatedmaterials containing BH₃ and amine compounds. The method involvesreacting a monohydrido boron compound with a selected ligand whereby themonohydrido boron compound disproportionates to a BH₃-containingcompound; and thereafter reacting the BH₃-containing compound withammonia.

The invention also includes a method of forming halo-boron compoundssuitable for reduction to boranes. The method involves reacting a boroncompound selected from the group consisting of alcoholato-boroncompounds, catecholato-boron compounds, amino-boron compounds, andanilino-boron compounds with a compound of the formula HX wherein X isselected from the group consisting of halogens; and thereafterseparating a product halo-boron compound.

The invention also includes a method of forming halo-boron compoundssuitable for reduction to boranes. The method involves reacting a boroncompound selected from the group consisting of alcoholato-boroncompounds, catecholato-boron compounds, amino-boron compounds, andanilino-boron compounds with an oxidizing agent, the oxidizing agentcomprising a corresponding halo-boron compound or halide salt of theboron compound; and thereafter separating a product halo-boron compound.

The invention also includes a method for synthesizing a BH-containingcompound. The method includes synthesizing at least one halo-boroncompound from a boron-containing precursor; and reacting the at leasthalo-boron compound with an inorganic hydride material, therebygenerating at least one B—H compound.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIGS. 1 a-d shows a schematic illustration of an embodiment for anenergy efficient preparation of diborane (B₂H₆). FIG. 1 a shows a partof the preparation where a silane (a material having a Si—H bond) isexposed to boron trichloride (BCl₃). FIG. 1 b shows the formation ofdiborane and the conversion of the silane to a chlorosilane (a materialhaving a Si—Cl bond), and FIG. 1 c-d shows the electrochemicalconversion of the chlorosilane back to the silane.

FIG. 2 shows a schematic illustration of an embodiment for an energyefficient preparation of (catecholato)BH using a polymer-supported tinreagent.

DETAILED DESCRIPTION

The present invention provides an energy efficient method forsynthesizing boranes (i.e. boron compounds having at least one B—H bond)that are used for storing hydrogen. These boranes are prepared withconsiderably less heat of reaction than present methods. Relatedbenefits are that boranes may be prepared at close to ambienttemperature without the need for thermal quenching and rapid separation,and without the energy cost of generating active metal hydrides. Spentborane-based hydrogen storage material could be regenerated inneighborhood facilities, and the regenerated material redistributed foruse as a transportation fuel. Using our invention, regeneration couldrequire electrical power as the only consumed resource. The inventionmay enable widespread use of boranes for hydrogen storage fortransportation.

The boranes synthesized by means of this invention may also be used asstarting materials for conversion to borohydride compounds forsubsequent use as chemical reducing agents or as chemical hydrogenstorage media.

FIGS. 1 a-d shows a schematic illustration of an embodiment of theinvention that relates to the preparation of diborane (B₂H₆). FIG. 1 ashows a part of the preparation where a silane (a Si—H material, i.e. amaterial with a silicon-hydrogen bond) is exposed to the halo-boroncompound boron trichloride (BCl₃). FIG. 1 b shows the formation ofdiborane and the conversion of the Si—H material to a halosilane (a Si—Xmaterial where X is a halogen). The halosilane shown in FIG. 1 b is achlorosilane (a Si—Cl material, i.e. a material having a Si—Cl bond).FIGS. 1 c-d show the electrochemical conversion of the halosilanematerial back to a silane. In this embodiment, electrochemical energyprovides the reducing potential to form the borane (diborane in thiscase), and a silicon-containing electrode essentially becomes anelectrocatalyst for the process. In principle the energy consumed isclose to the thermodynamic minimum required for forming boranes, andless energy is consumed using the invention than is used in knownmethods. This embodiment process approaches an efficiency that couldmake borane compounds practical for large-scale hydrogen storage fortransportation.

The silane used with the embodiment shown in FIGS. 1 a-d may be formedby, for example, immersing a high surface area silicon-containingelectrode in a liquid medium that contains electroactive hydrogen (H)atoms (water, for example) and under a cathodic potential sufficient toresult in the replacement of surface-capped groups on thesilicon-containing electrode with hydrogen. The electrode is then driedunder conditions such that the Si—H bonds remain on the surface, whichmay require exclusion of oxygen. The dry electrode is then exposed toBCl₃ as vapor or dissolved in an inert solvent. Diborane (B₂H₆) isformed and separated, and the silicon-containing electrode isre-hydrided.

It should be understood that other electrochemical reactions orprocesses may be used to convert halosilane back to silane.

It should also be understood that other materials that include but arenot limited to, zinc, gallium, germanium, indium, cadmium, mercury, andmixtures thereof, may be used instead of silicon.

It should also be understood that other halo-boron compounds (i.e.compounds of boron that contain at least one boron-halogen bond) besidesBCl₃ may be used. (Catecholato)BCl is an example of a halo-boroncompound that may be easier to prepare from spent borane fuel. Passing(catecholato)BCl over the hydrided silicon electrode (as describedpreviously for the BCl₃ embodiment) results in the conversion of(catecholato)BCl to (catecholato)BH. (Catecholato)BH may then bedisproportionated (using triphenylphosphine, diethylaniline, or someother reagent capable of promoting the disproportionation), resulting inthe formation of BH₃-containing compounds and B₂(catecholato)₃. Tocontinue the process, the B₂(catecholato)₃ is converted back to(catecholato)BCl. The conversion of B₂(catecholato)₃ back to(catecholato)BCl may be accomplished by, for example, reactingB₂(catecholato)₃ with HCl at elevated temperature and separating theproduct catechol from the product (catecholato)BCl. This conversion mayalso be accomplished by reacting B₂(catecholato)₃ with chlorine (oranother appropriate oxidant) to make (catecholato)BCl and quinone (anoxidation product of catechol).

Other halo-boron compounds that may be used include (amino)₂BCl and(amino)BCl₂ where “amino” is an organic group containing a primary orsecondary amine functionality bonded to the boron. Examples of such(amino)₂BCl compounds include but are not limited to (Me₂N)₂BCl,(piperidino)₂BCl, (NHCH₂CH₂NH)BCl, (o-NHC₆H₄NH)BCl. These halo-boroncompounds may react with a hydrided silicon electrode, or more generallywith compounds having Si—H bonds (or Sn—H bonds, or H atoms bonded tozinc, gallium, germanium, indium, cadmium, or mercury) to form(amino)₂BH compounds. The (amino)₂BH compounds may then bedisproportionated to form BH₃-containing compounds and (amino)₃Bcompounds. To continue the process the (amino)₃B compounds may beconverted back to (amino)₂BCl compounds by reacting them with HCl orwith other acids, or with chlorine or other oxidants, and thenseparating the product (amino)₂BCl compounds from the other reactionproducts, which may include the hydrochloride salt of the correspondingprimary or secondary amine.

FIG. 2 shows a schematic illustration of an embodiment of the inventionthat is concerned with preparing (catecholato)BH. In this embodiment, a(aryl)(dialkyl)(chloro)tin group attached to a polymer backbone isreacted with formic acid to produce the corresponding polymer-bound(aryl)(dialkyl)(formato)tin material, which is thermolyzed to form CO₂and the corresponding stannane, the polymer-bound(aryl)(dialkyl)(hydrido)tin material. (Catecholato)B—Cl reacts with thestannane to produce (catecholato)B—H and regenerate the polymer-bound(aryl)(dialkyl)(chloro)tin material.

In another embodiment, a stannane such as the polymer-supported tinreagent shown in FIG. 2 is used to convert BCl₃ to B₂H₆. The steps aresimilar: To convert BCl₃ to B₂H₆, the polymer-bound tin reagent isreacted with formic acid at room temperature to form hydrochloric acid(HCl) and the polymer bound (aryl)(dialkyl)(formato)tin material. TheHCl that forms is used to convert spent borane fuel to BCl₃. The(aryl)(dialkyl)(formato)tin material is heated to a temperature in therange of from about 120 degrees Celsius to about 180 degrees Celsius todrive decarboxylation, releasing CO₂ and forming polymer-bound(aryl)(dialkyl)(hydrido)tin material and CO₂. The CO₂ released isreacted with H₂ to provide formic acid. The polymer-bound(aryl)(dialkyl)(hydrido)tin material is reacted with BCl₃ to makediborane and regenerate the polymer-bound (aryl)(dialkyl)(chloro)tinmaterial. Because CO₂ and HCl can be recycled in the overall process,the net conversion is:spent borane fuel+H₂=fresh B—H fuel,and the polymer-supported tin materials serve as recyclable processintermediates. Overall the net conversion is endothermic and requiresenergy, which is provided by the endothermic decarboxylation of the(aryl)(dialkyl)(formato)tin material at 120-180 degrees Celsius. Thisembodiment provides a significantly better process than the directhydrogenation of BCl₃, which requires heating to about 600 degreesCelsius.

While FIG. 2 shows a single embodiment based on a polystyrene backbone,it should be understood that other polymer backbones may be used withthe invention, including but not limited to, other polyalkenes(polyethylene, polypropylene, for example) polyethers, ROMP (ringopening metathesis polymerization) polymer products of cycloalkenesmonomers, polysulfides, polyphosphazenes, polyborazenes, polyanilines,polysilanes, and branched, cross-linked, and alkyl- and aryl-substitutedpolymers and copolymers of these materials (cross-linked polyethylene,for example).

In a more general embodiment, halo-boron compounds (preferably B—Clcompounds) are reacted with stannanes (tributyltin hydride, for example)to convert the halo-boron compound to a borane. The stannane isconverted to a halo-stannane (a tin compound having a tin-halogen bond,preferably a Sn—Cl bond). The stannane is regenerated by reacting thehalo-stannane with an inorganic metal hydride. The stannane also can beregenerated by electrochemical means similar to the silicon-based route(vide supra), or by reacting the halo-stannane with formate to form aSn-formate compound and then subsequently thermolyzing the Sn-formatecompound.

Other stannanes (hydrided tin-containing electrodes, for example) mayreact with one or more halo-boron compounds to convert the halo-boroncompounds to boranes and halostannanes. The halostannanes may bere-hydrided by formate exchange and thermolysis as above, or byelectrochemical means (similar to the silicon-based route above) to formthe stannanes.

Additional embodiments are provided in the following EXAMPLES.

EXAMPLE 1

Triethylsilane (0.2 mL) was added to a solution of BCl₃ (0.2 g BCl₃ in 2mL of CD₂Cl₂). The resulting solution was monitored by nuclear magneticresonance (NMR) spectrometry. After 3 minutes all of the BCl₃ hadreacted. The only boron-containing product present was B₂H₆.

EXAMPLE 2

Tributyltinhydride (0.3 g) was added to a solution of BCl₃ (0.2 g in 2mL of CD₂Cl₂). The resulting solution was monitored by NMR. After 30minutes all of the BCl₃ was consumed and the only boron-containingproduct present was B₂H₆.

EXAMPLE 3

A freshly prepared hydrided silicon surface was exposed to a solution ofBCl₃ in hexanes. The solution was left in contact with the siliconsurface for 30 minutes. Infrared analysis of the silicon surfaceindicated that the silicon hydrides were converted to silicon chlorides.By inference, the boron chloride was concerted to boron hydride.

EXAMPLE 4

A freshly prepared hydrided silicon surface was exposed to a solution ofBBr₃ in hexanes. The solution was left in contact with the siliconsurface for 30 minutes. Infrared analysis of the silicon surfaceindicated the conversion of the silicon hydrides to silicon bromides. Byinference, the boron bromide was converted to boron hydride.

EXAMPLE 5

In a hypothetical example, an aminoborane is dehydrogenated and combinedwith a catechol ligand to form a product. This product is dissolved in asolvent to form a solution. Tributyltinhydride is added to the solution.There is a metathesis reaction between the tributyltinhydride and thisdissolved product to form boron hydrides.

EXAMPLE 6

In a hypothetical example, an aminoborane is dehydrogenated and combinedwith a catechol ligand to form a product. This product is dissolved in asolvent to form a solution. Triethylsilane is added to the solution.There is a metathesis reaction between the triethylsilane and thisdissolved product to form boron hydrides.

EXAMPLE 7

In a hypothetical example, an aminoborane is dehydrogenated and combinedwith a catechol ligand to form a product. This product is dissolved in asolvent to form a solution. A freshly hydrided piece of silicon is addedto the solution. There is a metathesis reaction between the siliconhydride on the surface and this dissolved product to form boronhydrides.

EXAMPLE 8

In a hypothetical example, an aminoborane is dehydrogenated and combinedwith a catechol ligand to form a product. This product is dissolved in asolvent to form a solution. A piece of titanium with a hydride surfaceis added to the solution. There is a metathesis reaction between thetitanium hydride and this dissolved product to form boron hydrides.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A method for synthesizing a BH₃-containing compound, comprising:synthesizing at least one halo-boron compound from a boron-containingprecursor; reacting the at least one halo-boron compound with aninorganic hydride material, thereby generating at least one B—Hcompound; and disproportionating the at least one B—H compound to atleast one BH₃-containing compound.
 2. The method of claim 1, furthercomprising the step of forming an inorganic hydride material withhydride species on the surface of the inorganic hydride material by anelectrochemical reaction before the step of reacting the at least onehalo-boron compound with the inorganic hydride material.
 3. The methodof claim 1, wherein the inorganic hydride material comprises a polymericmaterial comprising silicon, tin, zinc, gallium, germanium, indium,cadmium, mercury, or mixtures thereof, where a population of hydridespecies is formed by reaction with another metal hydride or by exchangewith formate and subsequent thermolysis.
 4. The method of claim 1,wherein the inorganic hydride material comprises silicon, tin, zinc,gallium, germanium, indium, cadmium, mercury, or mixtures thereof. 5.The method of claim 1, wherein the inorganic hydride material comprisesan electrode.
 6. The method of claim 4, wherein the inorganic hydridematerial comprises an electrode.
 7. The method of claim 1, wherein theat least one halo-boron compound comprises a chloro-boron compound.
 8. Amethod for making a metal hydride, comprising thermolyzing a reactivemetal formate.
 9. The method of claim 8, wherein the metal hydride is acompound of the formula(R′)_(n)(R″)_(m)(X)_(3-n-m)Sn(H) wherein R′ is alkyl, wherein R″ is arylor aryl attached to a polymer backbone, wherein X is F, Cl, Br, or I;wherein n is 0, 1, 2, or 3; wherein m is 0, 1, 2, or 3; and whereinn+m≦3.
 10. The method of claim 8, wherein the metal hydride is acompound of the formula selected from the group consisting of R₃SnH,R₂XSnH, RX₂SnH, and X₃SnH, wherein R is selected from alkyl and aryl,and wherein X is selected from halogen.
 11. A method of forming BH₃NH₃and related materials containing BH₃ and amine compounds, comprising:reacting a monohydrido boron compound with a selected ligand whereby themonohydrido boron compound disproportionates to a BH₃-containingcompound; and thereafter reacting the BH₃-containing compound withammonia.
 12. A method of forming halo-boron compounds suitable forreduction to boranes, comprising: reacting a boron compound selectedfrom the group consisting of alcoholato-boron compounds,catecholato-boron compounds, amino-boron compounds, and anilino-boroncompounds with a compound of the formula HX wherein X is selected fromthe group consisting of halogens; and thereafter separating a producthalo-boron compound.
 13. A method of forming halo-boron compoundssuitable for reduction to boranes, comprising: reacting a boron compoundselected from the group consisting of alcoholato-boron compounds,catecholato-boron compounds, amino-boron compounds, and anilino-boroncompounds with an oxidizing agent, the oxidizing agent comprising acorresponding halo-boron compound or halide salt of the boron compound;and thereafter separating a product halo-boron compound.